Introduction
The name Coastal Group (after Coastal Series of Hill 1899) is used for the various limestones, impure limestones, dolostones, marlstones and clastics (sandstones and conglomerates) that were deposited around the margin of Jamaica during its tectonic uplift over the last 12-14 million years. Although many of these units are shallow-water deposits with abundant corals, there are also deep-water marlstones and even some igneous rocks.
The Coastal Group can be divided into an older part (mid/late Miocene to early Plesitocene) and a younger part (late Pleistocene). This is useful for descriptive purposes and also reflects an important tectonic event in the mid Pleistocene.
In the eastern part of the island, the older part of the Coastal Group consists of several formations.
---- August Town Formation (Miocene - early Pleistocene). A series of shallow-water impure limestones that are found in the coastal areas of the parishes of Manchester, Clarendon, St Catherine, St Andrew and St Thomas. In some areas the August Town Formation occurs at the base of the succession (i.e., below the Layton Formation -- e.g., in St Thomas), whereas elsewhere (Clarendon and St Andrew) much of the mid/late Miocene to Pleistocene may be made up of the August Town Formation.
---- Low Layton Formation In northern Portland, around Black Hill, a series of pillow basalts is found between the sediments of the White Limestone and the Coastal Group. These rocks contain vesicles, some of which are filled with limestone. The Low Layton basalts were therefore formed in relatively shallow water (otherwise they would not have vesicles), and indicate an episode of local extension that opened up fissures allowing for the eruption of the basalts.
---- Layton Formation (late Miocene to Pliocene). This consists of marlstones and contains the famous Bowden Shell Bed at Bowden in St Thomas. In St Thmoas, the Layton Formation rests on the August Town Formation, but in Portland the Layton Formation rests on chalks of the Spring Garden Formation (White Limestone Group). In Portland, the Layton Formation can be divided into three members (Buff Bay Member, San San Member, and Bowden Member) based on unconformities picked up from biostratigraphical studies. These members are not lithologically distinct, and the name formation is unappropriate for them. In St Thomas, the Buff Bay Member, and maybe the San San Member, are not developed, and in this area the term Bowden Formation could be used (this is a case of a unit changing rank from member to formation as it is mapped out).
---- Manchioneal Formation (Early Plesitocene). In eastern St Thomas and eastern Portland, the Layton (Bowden) Formation is succeeded by the Manchioneal Formation, which consists of impure limestones that locally contain pebbles beds (conglomerates) and sandstones. In many ways, the Manchioneal Formation is lithologically very similar to the August Town Formation. In St Mary, the lower Coastal Group is represented solely by impure carbonates and the deep-water marlstones of the Layton Formation are absent. Here, this part of the Coastal Group has been mapped as Manchioneal Formation rather than August Town Formation. Similar impure facies of the lower Coastal Group occur west of Montego Bay in St James and in north-western Hanover, and contrast with the hard (often dolomitized) white limetones of the Hopegate Formation.
---- Old Pera Formation (Early Pleistocene). Around Old Pera in St Thomas, the Manchioneal Formation changes facies and is represented by a series of shelf sandstones. These have been mapped as the Old Pera Formation, but have a relatively restricted distribution in this part of the island.
---- Harbour View Formation (Pleistocene?). In St Andrew, the August Town Formation is succeeded by clastic rocks (mostly conglomerates, although there are a few thin impure limestones), which are well-developed on the southern end of Long Mountain and on the western side of Dallas Mountain. Similar conglomerates also occur above the August Town Formation at Round Hill in Clarendon, but have a different provenance for their clasts (those in the Harbour View Formation at Harbour View contain igneous and sedimentary rocks derived from the Wag Water Belt together with pebbles of White Limeston; whereas those in Clarendon are derived from the Summerfield Group of the Central Inlier).
In the northwestern part of Jamaica, the older part of the Coastal Group has been called the Hopegate Formation (now attributed to the Early Pleistocene) and contains abundant reef-building corals and is often dolomitized. From St Mary to Westmoreland, different intervals of the coastline are characterised by Hopegate-like or Manchioneal-like facies, facies-distinctions that were probably controlled by availability of clastic detritus (sands and clays) supplied from the tectonic emergence of the Cretaceous inliers (Sunderland and Lucea inliers) as Jamaica was uplifted.
The upper part of the Coastal Group is represented by the Falmouth Formation ('raised' coral reefs and associated lagoonal depoists) and Port Morant Formation (a disected fan delta).
You can find some papers relating to the Coastal Group here:
Donovan, S. K. and Miller, D. J. 1999. Report of a Field Meeting 23rd May 1988. Journal of the Geological Society of Jamaica Volume, 33, 31-41. GO TO SITE
James-Williamson, S. A. and Mitchell, S. F. 2012. Revised lithostratigraphy of the Coastal Group of south-eastern St. Thomas, Jamaica. Caribbean Journal of Earth Science, 44, 9-17. GO TO SITE
James-Williamson, S.A., Mitchell, S.F. and Ramsook, R. 2014. Tectono-stratigraphic development of the Coastal Group of south-eastern Jamaica. Journal of South American Earth Sciences, 50, 40-47. GO TO SITE
Mitchell, S. F., Pickerill, R. K., Blackwell, B. and Skinner, A. R. 2000. The age of the Port Morant Formation, southeast Jamaica. Caribbean Journal of Earth Science, 34, 1-4. GO TO SITE
Mitchell, S. F., Pickerill, R. K., and Stemann, T. 2001. The Port Morant Formation (Upper Pleistocene, Jamaica): high resolution sedimentology and palaeoenvironmental analysis of a mixed-carbonate-clastic lagoonal succession. Sedimentary Geology, 144, 291-306. GO TO SITE
Pickerill, R. K., Donovan, S. K. and Mitchell, S. F. 1998. Ichnology of the Late Pleistocene Port Morant Formation of southeastern Jamaica. Caribbean Journal of Science, 34, 12-32. GO TO SITE
Pickerill, R. K., Mitchell, S. F., Donovan, S. K. and Keighley, D. G. 1998. Sedimentology and palaeoenvironment of the Pliocene Bowden Formation, Southeast Jamaica. Contributions to Tertiary and Quaternary Geology, 34, 12-32. GO TO SITE